Discuss
On a private mailing list, someone posted a
couple
of links
about recent progress towards the development of a quantum computer,
with the comment: Quantum computers provide irresistible evidence
that the multiverse is real. I replied to the list:

J.W.Burton will correct me if I'm wrong, I'm sure. Quantum computers
do nothing of the kind. They provide yet another experimental proof
(and exploitation of) complementarity and non-locality. Whether the
nature of the universe that underlies these characteristics -- the
quantum reality -- is as described by Bohr (the moon is really not
there when no-one's looking at it) or by Bohm (a new universe
branches from every quantum choice), or none of the above, is an
open question.

The authority invoked in the first sentence is Joshua W. Burton,
itinerant professional physicist and one of the more cogent voices
on that list, or any other for that matter. Dr. Burton rose to my
troll, as I had hoped he would, with the following essay. It's the
most compelling summary that one could desire of the current state
of thinking about quantum reality, and it is 100% guaranteed to make
your head hurt.

At some level, this is an aesthetic question. Galileo and Kepler
did not prove that the sun does not revolve around the earth
-- indeed, since we now know that the Einstein equations (G = 8pi T,
of course, not E_0 = m) have general covariance, we know that no
such thing can ever be proved, and that we are free to believe
earth, moon and sun are simultaneously stationary if we have
some good reason to work in such twisted coordinates.

I agree that quantum computers offer nothing new beyond a cool
exploit of Hilbert space projection.* Whether quantum computers
offer lots of new computational light or just a handy flashlight for
a few dark corners known classical algorithms have missed, they say
nothing about quantum mechanics that we haven't known since
Bell's
inequality was proved in the early 1960s.

However, at some point aesthetic considerations in physics move
beyond the realm of fashion and become compelling in the minds of
most working physicists. I think the cited article is essentially
correct in noting that quantum computers (along with inflationary
cosmology) have been influential in convincing most practicing
theoretical physicists that Everett sum-over-histories, or sometimes
Cramer's so-called transactional interpretation, are more fruitful
ways to look at QM than either Bohm's curious picture or the old
Bohr / Heisenberg / Schroedinger mess usually called
"the
Copenhagen interpretation." (Once in England I saw a sign that
said "railway closed due to labour action," and remember thinking,
"No it's not, it's closed due to labor inaction." That's the
sense in which Bohr had an interpretation of QM.)

For those who don't have mental images to go with the names, here is
the best Hillel synopsis (or elevator story, as they now say in
Dilbert land) I can supply:

Everybody, thanks to Bell's theorem and Aspect's experiment:
QM cannot simultaneously be local, deterministic, and real.
(Local: no spooky action-at-a-distance-faster-than-c.
Deterministic: no true randomness in future, if you know present.
Real: no divine crooked three-card-monte, where all the contradictory
experiments are ones you just didn't happen to do this time.)

Bohm: Measurements are deterministic, nonlocal events.
Only wavefunctions, not actual particles, behave nonlocally,
fortunately. Since relativity can't be formulated nonlocally, let's
give up on field theory for now.

Penrose: Measurements are local, nondeterministic events.
They only happen in brains, and probably not in
Doug Hofstadter's.

[If someone has had the guts to suggest that it's realism
which has to give, I haven't heard tell. That would be
Leibniz's
monads, in essence.]

Everett: Measurements are superstitions. Unitary QM rules
at all times, and if you decide to project your own wavefunction
into "what happened" and "what didn't happen" subspaces, that's your
problem.

Cramer: Same as Everett, except for asking questions with
a beginning and ending boundary condition, rather than just a
beginning and open time evolution from there. Sum-over-histories =
Everett + Cramer.

(Note that I'm not claiming that any of these people actually said
what I'm hanging on their names. It's just that these views are, as a
sociological matter, so denoted by practicing physicists, who have a
spotty record at best for historical accuracy in their myth-building.)

No one anywhere is arguing about the math, only about what it means.
Sum-over-histories allows you to ask questions (like what boundary
condition to put on the universe's wavefunction near the Big Bang, or on
the infalling star's wavefunction inside a black hole) that the others
can't even formulate; it's more powerful than Copenhagen in roughly the
same way that Bayesian statistics are more powerful than classical, but
when both can ask the same question, they (provably) get the same answer.

Anyway, quantum computing no more proves many-worlds than the phases of
Venus prove heliocentrism. But in both cases the new facts eventually do
direct the strokes of Ockham's razor.

*

The Shor
algorithm is a lot more specific than most people
realize. Quantum computers happen to be able to achieve rapid
prime factorization, but there is really no very strong reason
to believe that prime factorization was hard, and you almost
certainly can't leverage Shor to solve NP-hard problems. A
few of my brightest friends suspect that P != NP may be proved
within our lifetimes by showing that P <= QP << NP, simply
because the class of problems that quantum computers can solve
in polynomial time may be easier to constrain than classical P.